The present invention relates to a system for laying subsea pipelines and a method by which such system can be laid.
In particular, the present invention is primarily concerned with laying pipelines that are relatively small, e.g., up to approximately six inches in inner diameter, and relatively short, e.g., up to approximately two miles. The system of the present invention is intended for use in laying pipelines in either shallow or deep water.
A factor that must be considered in laying pipelines from a vessel moving along the water is the necessity for maintaining a proper catenary to the pipeline between the water surface and the seabed. Care must be taken to avoid stressing the pipeline excessively otherwise buckling, also known as kinking, of the pipeline will occur as it is paid off the vessel into the water. One way to minimize possible buckling problems especially in deep-water laying operations is to use a pipe-handling mechanism that allows for a variable angle of entry; see, for example, U.S. Pat. No. 3,982,402 to Lange. Alternatively, a stinger appended to the stern of the vessel can be utilized to support the pipe in the water; see, for example, U.S. Pat. No. 3,685,305 to Lloyd. Maintenance of a proper catenary is obtained by controlling both the tension on the pipe and the angle of inclination that the pipe forms with the seabed as it is laid. Significant research has been done to avoid or at least minimize the impact of such problems. This research has led to the development of relatively complex systems that also involve extremely complicated operations.
While a plurality of systems have been developed to resolve such problems, those developments generally involve complex systems for avoiding overstressing of the pipeline as a line is being paid out from the vessel on the water surface. Such systems are concerned with maintaining proper tension on a pipeline and controlling the angle of inclination of the pipeline as it is being fed out. While this problem is primarily one experienced in deep-water laying of pipelines, the same type of problem also occurs in laying pipelines in relatively shallow water although the problem may not be as severe.
While the majority of the developments for avoiding the above noted problems have been involved with the provision of complex mechanisms for controlling the feeding of the pipeline from the water surface, one other system is known which takes a different approach for solving the same type of problem. In this system, the pipeline is wound of a reel which is supported by some type of structure that enables the reel to be pulled along the seabed. The free end of the pipeline is then attached to the wellhead and the reel is then pulled by a seagoing vessel towards the termination point. While in theory, this type of system avoids the previously discussed problems, other problems of a similar nature are encountered. Since the reel is relatively heavy it is difficult to control the speed at which it is pulled towards the termination point by the vessel on the surface. If the reel is pulled too rapidly, excessive stress is applied to the pipeline which can result in damage to the line. In addition, any jogging of the reel as it is moved along the seabed due to either a change in the contour of the bed or the motion of the towing vessel can result in kinks being formed in the pipeline. Consequently, the same type of damage that can be incurred in laying pipeline from the water surface also can result with the use of this type of system.